GB2518304A

GB2518304A - Engine starting system

Info

Publication number: GB2518304A
Application number: GB1417611.9A
Authority: GB
Inventors: Stephen Clarke Baker; Jerry Elphick; Jeffrey Charles Moore
Original assignee: MBE SYSTEMS Ltd
Current assignee: MBE SYSTEMS Ltd
Priority date: 2013-09-17
Filing date: 2013-09-17
Publication date: 2015-03-18
Anticipated expiration: 2033-09-17
Also published as: GB201316517D0; GB2518238A; GB201417611D0; GB2518304B; GB2518238B

Abstract

A starting system for an engine comprises a sensor means suitable for sensing the speed, position and direction of the engine, and a control unit adapted to receive speed, position and direction information from the sensor means and to disable fuel supply and/or ignition to the engine if the engine is rotating in a reverse direction. The sensor means includes a toothed wheel 10 attachable to the crankshaft. The position of the toothed wheel 10 may be provided by a "signature tooth" equivalent to two normal teeth with the gap between them filled in. Two detectors 16,18, eg sensors may be Hall effect sensors or variable reluctance sensors, are positioned adjacent the wheel 10. The angle Î¸d between the detectors may be an integer and a half times the tooth angle Î¸t. The control unit is adapted to determine speed and position information from the sensor means when one of the two detectors is not operational. The sensor means is particularly suitable for use on an engine in an unmanned aerial vehicle, where it allows the engine to be started by a small motor and power source, protecting against kickback.

Description

ENGINE STARTING SYSTEM

The present invention relates to a starting system for an engine, and particularly but not exclusively to a starting system for an aircraft engine, especially a starting system for an engine in an unmanned aircraft.

BACKGROUND TO THE INVENTION

During normal mnning of a spark-ignition engine, a spark in the combustion chamber will fire towards the end of the compression stroke. In a piston-cylinder engine, this is before the piston reaches top dead centre. Advancing the ignition timing before top dead centre ensures that the fuel burns completely, and transfers the maximum amount of mechanical energy during the power stroke, A spark which fires too late can result in incomplete combustion, lack of power and poor fuel economy.

On the other hand, a spark which fires too early during the compression stroke can cause multiple flame fronts in the combustion chamber. This is known as "knocking", and can damage an engine because it causes excess vibration. It is therefore vital to get the ignition timing exactly right. Modem engine control units will automatically adjust ignition timing as engine speed increases and decreases, so that the timing is advanced as far as it can be, to provide optimum performance and fuel economy, without knocking. A knock sensor can also be provided in the engine, to detect knocking as soon as it occurs, The engine control unit can then retard the ignition timing to mitigate the problem before it causes serious damage.

In order to determine the correct time to fire the ignition, the engine control unit needs an accurate input of the current speed and position of the engine. A crankshaft sensor provides this information, and typically comprises a hall-effect sensor mounted adjacent to a toothed wheel on the crankshaft. In this way, a modern electronically controlled spark-ignition engine runs efficiently and reliably, by dynamically adjusting ignition timing during operation, However, there is still room for improvement in the performance of engines. A particular problem can occur when the engine is being started, An engine is generally started by an external drive (for example, powered by an electric motor) which turns the engine substantially more slowly than the normal running speed of the engine.

Furthermore, the actual speed of the starter may be variable, depending on factors such as the condition of the battery.

As a result of these non-ideal and uncertain conditions, there is an increased risk during starting that the ignition timing will not be optimal. To compound this, the damage caused by a premature ignition during starting is potentially much greater than the damage caused by premature ignition during normal running. Because of the reduced momentum in the engine, an ignition event too soon before the end of the compression stroke may cause the piston to reverse in the cylinder, and the engine to run in the wrong direction. This phenomenon is known as kickback', and can cause serious damage. The engine running in reverse may easily overcome the torque supplied by the starter in a single power stroke, and run the starter backwards, potentially destroying the starter and/or the starter drive clutch arrangement.

Some small unmanned aircraft engines are started with an external ground-powered starter motor, which is applied to the engine to start and then removed. Kickback in this scenario may injure the operator. Similarly, kickback often causes rider injury in kick-start motorcycles. It is possible to reduce the risk of kickback when manually starting an engine by carefully rotating the engine to the beginning of the power stroke before starting. However, this relies on operator skill and experience to reduce the risk of injury.

An aircraft engine usually drives a propeller without any clutch arrangement Therefore, if the engine actually starts in reverse and continues to run, then unwanted and potentially dangerous operation of the propeller in the reverse direction will result.

The risk of kickback is currently reduced by using very conservative (i.e. heavily retarded) ignition timings when the engine is started. However, even with safe' settings, kickback can still occur when the torque from the starter is substantially below what is expected, for example because the battery is depleted. Only by retarding the ignition until after the piston reaches top dead centre can the risk of kickback be eliminated altogether, and this results in incomplete combustion and greater pollution. Since the energy transferred to the engine during the power stroke is reduced, it also makes the engine more difficult to start.

To reduce the risk of kickback without excessively retarding ignition timings, all oversized starter motor and battery can be used. By ensuring that a large amount of torque is delivered to the engine on starting, the ignition timings can be safely set to start the engine cleanly and efficiently. However, although providing a large battery ensures a greater safety margin, there is always a risk that a battery of whatever size can become depleted.

An over-specified starter and battery adds to the weight of a vehicle, This is particularly problematic for aircraft, Where an external ground-powered manual starter is applied to an unmanned aircraft before take-off weight on the aircraft is not a problem. However, it would be preferable to provide an on-board starter, because this would allow the engine to be stopped and restarted during flight, enabling greater fuel economy as the aircraft is able to glide, unpowered, for parts of its journey, However, providing a starter and battery small and light enough to be carried in flight reduces the available starting torque, increasing the risk of kickback as described above.

Although the problem of a small starter motor being used to start an engine is particularly relevant to unmanned aircraft, it is also present in some motorcycles.

Many motorcycles have single-or twin-cylinder engines, and therefore a compression stroke in one cylinder is not balanced by an expansion stroke in another cylinder, as would be the case in, for example, a car engine, As a result, single-and twin-cylinder engines require a relatively high starting torque for their size. However, space is limited on motorcycles for the large motor and battery which are required to provide this.

Before an electronically controlled engine can be started, the engine control unit must have correctly calculated an accurate value for the engine position, This is necessary so that fuel can be injected, and/or spark plugs can be fired, at the correct times.

Typically, engine position is determined by a signature tooth on the toothed wheel of the crankshaft sensor. The signature tooth is typically equivalent to two adjacent normal teeth, with the gap between the two teeth filled in. The crankshaft must therefore be rotated by 360 degrees before it can be guaranteed that the signature tooth will pass the Hall probe, giving the engine control unit an accurate measurement S of engine position and allowing ignition / fuel injection to take place. This increases the time taken to start an engine, which increases battery drain on the starter and is a particular disadvantage where stop-start operation is used to realise greater fuel economy, since delay in starting can result in effective loss of control, especially for an aircraft.

Although the background is described in terms of spark-ignition engines, analogous problems are present in compression-ignition engines. In the case of a compression-ignition engine, it is the fuel injection which will cause inefficient and unclean operation if it occurs too late, and a risk of kickback if it occurs too eaiJy.

It is an object of the invention to reduce or substantially obviate the above-mentioned problems.

STATEMENT OF INVENTION

According to the present invention, there is provided a starting system for an engine, the starting system comprising a sensor means and a control unit, the sensor means being suitable for sensing the speed, position and direction of the engine, the sensor means including: a toothed wheel attachable to a crankshaft of the engine, a first detector mounted in proximity to the toothed wheel, for detecting whether or not a tooth of the wheel is adjacent the first detector; and a second detector mounted in proximity to the toothed whe&, for detecting whether or not a tooth of the wheel is adjacent the second detector, and the control unit being adapted to receive signals from the sensor means and to disable fuel supply and/or ignition to the engine if the engine is rotating in a reverse direction.

The two detectors allow for sensing of direction, as well as speed and position, by the sensor means.

The teeth of the wheel may have first and second edges. In one direction, the first edge of each tooth will be a leading edge and the second edge will be a trailing edge, and in the other direction, vice versa.

Preferably, the detectors are disposed around the wheel such that when the first sensor is adjacent a first edge of a tooth, the second sensor is adjacent a tooth (i.e. between the leading and trailing edges of a tooth), and when the first sensor is adjacent a second edge of a tooth, the second sensor is adjacent a gap between teeth (i.e. between the trailing edge of one tooth and the leading edge of the next tooth in the direction of rotation), the second sensor not being adj acent an edge of a tooth when the first sensor is adjacent an edge of a tooth. In other words, the teeth and the gaps between teeth form peaks and troughs, When a sensor is between teeth it is adjacent a trough, and when a sensor is adjacent a tooth it is adjacent a peak.

Ideally, then position of the sensors would be such that when the first sensor is adjacent the first edge, the second sensor is always adjacent a tooth, and when the first sensor is adjacent a second edge, the second sensor is always adjacent a gap between teeth. However, it is possible to have a signature tooth on the wheel, as described below, and the relative positions of the first and second sensors and the edges of the teeth may not be as described above in the specific case when the signature tooth is adjacent one of the sensors. However, this will not have a serious impact on the ability to detect direction -the control unit may just have to wait until the wheel has rotated a little past the signature tooth to correctly detect direction.

In this way, the control unit may determine the direction by measuring the output of the second sensor at the moment when the first sensor transitions from a "tooth present" output to a "tooth absent" output (i.e. when a trailing edge of a tooth passes the first sensor), or by measuring the output of the second sensor at the moment when the first sensor transitions from a "tooth absent" output to a "tooth present" output (i.e. when a leading edge of a tooth passes the first sensor).

In a most preferred embodiment, the teeth are equally spaced around the wheel, with the gaps between the teeth being equal to the width of the teeth. The ideal position for the first and second sensors is with the sensors spaced from each other by an integer and a half times the tooth width.

The control unit can use the direction information to detect a kickback, and immediately cut off the fuel supply and disable the ignition. Further remedial action could also be taken. For example, an exhaust port could be opened to remove pressure caused by the initial ignition event from the piston of the engine, and the starter drive could be disengaged to reduce the possibility of damage.

By detecting kickbacks and taking immediate remedial action, the potential for damage to the engine and/or injury to the operator is substantially reduced. As a result, ignition timing may be advanced to provide a cleaner and more efficient start.

Also, a smaller starter motor and/or battery may be utilised, which is particularly advantageous in applications where space or weight is at a premium, for example on aircraft and on motorcycles.

On a small unmanned aircraft, the starter system which detects kickbacks allows for a small starter to be carried on board, where previously such engines were started by an external ground-powered starter, As a result the engine may be stopped and restarted during flight, allowing for periods of unpowered gliding, substantially increasing the overall fuel economy of the aircraft.

On a single-or twin-cylinder motorcycle engine, safe starting is possible with a small starter motor or with a kick-start, despite the increased starting torque which is required to reliably start this type of engine.

For position sensing, a signature tooth may be provided, which is for example double or triple the angular width of the remaining teeth on the wheel. It is possible to detect speed, position, and direction separately, but preferably a single toothed wheel is provided with a signature tooth and detectors positioned as described above, to provide speed, position and direction sensing with minimum part count.

In aircraft engines, multiple redundant components are required to be installed in order to provide safety backup in case of failure. Therefore two detectors can be provided without any additional weight cost or increased part count as compared to existing engines, which have two detectors in the crankshaft sensor anyway for redundancy. By positioning the detectors in the manner described, they provide the additional benefit of direction sensing, as well as the speed and position sensing which is already provided by one detector, or is provided with a redundant backup by two detectors, Speed and position sensing is used for correct ignition and/or fuel injection timing by existing engine control units.

Extra detectors are not required to provide a safety backup for direction sensing, since direction sensing is only important during starting. Should one detector fail during flight, then the engine can be kept running continuously, as with current unmanned aircraft which have no on-board starter. There is therefore no reduction in safe levels of redundancy as compared to current equipment, and also no increase in the number of components which are required.

Typically, the detectors output an electrical signal which can be interpreted by the control unit, The detectors may be Hall effect sensors, variable reluctance sensors, or any other detector which detects the presence or absence of a tooth at a particular position around the circumference of the toothed wheel, Preferably, the two detectors are positioned about 180 degrees apart, or as close as possible to 180 degrees within the above mentioned constraint that the detectors must be spaced by an angle corresponding to an integer and a half times the distance between teeth. This means that the engine only has to be rotated by 180 degrees in order to guarantee that the signature tooth on the wheel will pass one or the other of the detectors, giving the control unit an accurate measurement for the position of the engine, As such, the engine can be started more quickly than with existing speed and position sensors, which may require a full 360 degree rotation of the crankshaft sensor to determine engine position, The starting system may be applied to many types of engine, and in particular is suitable for use in spark-ignition and compression-ignition engines, and rotary and piston-cylinder engines.

S DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it maybe carried into effect, reference will now be made by way of example only to the accompanying drawings, in which: Figure 1 shows a schematic perspective view of a toothed wheel and a pair of detectors, forming part of an engine starting system according to the invention; Figure 2 is a diagram showing the detailed position of the detectors relative to the toothed wheel of Figure 1; Figure 3a is a diagram showing the output from the detectors of Figure t, when the toothed wheel is rotating in a first direction; and Figure 3b is a diagram showing the output from the detectors of Figure 1, when the toothed wheel is rotating in the opposite direction to the direction of rotation in Figure a.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring firstly to Figures 1 and 2, a toothed wheel is indicated generally at 10, In use, the toothed wheel 10 may be aftached to the crankshaft of an engine, so that the wheel 10 rotates with the crankshaft.

A plurality of teeth 12 are disposed around the circumference of the wheel to.

Although in the Figure teeth are shown around only a portion of the circumference, in the preferred embodiment teeth are provided all the way around the wheel 10.

S

The teeth 12 are generally equally sized and equally spaced around the wheel 10.

However, a double signature tooth' 14 is essentially two norma' teeth 12 with the gap between the teeth filled in, i.e. it is three times the width of the other teeth 12. The signature tooth 14 provides a reference position of the toothed wheel 10, and hence S the crankshaft. It therefore allows for the position of the engine to be determined (ic.

in a piston cylinder engine, which cylinders are in which position in which strokes.) Two Hall effect detectors 16, 18 are positioned adjacent the wheel 10, for detecting the presence or absence of a tooth at the location of the respective detector at any particular moment.

As seen in Figure 2, the teeth are substantially equally spaced, the angle between teeth being the same as the angle occupied by each tooth. The angle occupied by a tooth 12 is indicated as e1. The positions of the Hall effect detectors are indicated in Figure 2 by arrows 16, 18, and the angle between the detectors is indicated as °d The angle between the detectors 6d is an integer and a half times the tooth angle O. In Figure 2, arrow A indicates the normal (i.e. forwards) direction of rotation. Arrow B indicates the reverse, undesirable, direction of rotation, Figure 3 a shows the output from the detectors 16 and 18 when the whe& 10 is rotating in the direction of arrow A, The output of a detector 16, 18 is high when a tooth 12 is adjacent the detector, and low when no tooth 12 is adjacent the detector, When the output transitions 22 from low to high, this indicates that a leading edge of a tooth has passed the detector.

By monitoring the state of second detector 18 when the first detector 16 transitions 22 from low to high, the direction of rotation can be determined. As seen in Figure 3a, the second detector 18 is ow on the transition 22 of the first detector when the wheel 10 is rotating in the direction of arrow A, As seen in Figure 3b, the second detector 18 is high on the transition 22 of the first detector when the wheel 10 is rotating in the direction of arrow B, Likewise, on a high to low transition 24 of the first detector 16, the second sensor 18 is high when the wheel 10 is rotating in direction A, and low when the wheel 10 is rotating in direction B, Also, the direction can be determined on a low to high or high to low transition of the second sensor 18, by reference to the output of the first sensor 16. Where the angular width of each tooth is the same as the angular width of the gaps between teeth, the S wheel 10 needs to rotate through only half of that angle in order for the direction of rotation to be correctly determined by a control unit receiving the output of sensors 16, 18.

The arrangement of detectors 6, 18 around the toothed wheel 0 allow direction of rotation to be determined. As a result, dangerous and damaging kickback events can be detected by an engine control unit as soon as they occur, and remedial action may be taken, This mitigates the risk of kickbacks which is associated with using a small starter with a large engine.

Claims

CLAIMSI. A starting system for an engine, the starting system comprising a sensor means and a control unit, the sensor means being suitable for sensing the speed, position and direction of the engine, the sensor means including: a toothed wheel attachable to a crankshaft of the engine; a first detector mounted in proximity to the toothed wheel, for detecting whether or not a tooth of the wheel is adjacent the first detector; and a second detector mounted in proximity to the toothed wheel, for detecting whether or not a tooth of the wheel is adjacent the second detector, and the control unit being adapted to receive speed, position and direction information from the sensor means, and to disable fuel supply and/or iñtion to the engine if the engine is rotating in a reverse direction, in which the control unit is adapted to determine speed and position information from the sensor means when one of the two detectors is not operational.
2. A starting system as claimed in claim 1, in which each tooth has a first edge and a second edge.
3. A starting system as claimed in claim 2, in which the detectors are positioned relative to one another such that when the first sensor is adjacent a first edge of a tooth, the second sensor is adjacent a tooth, and when the first sensor is adjacent a second edge of a tooth, the second sensor is adj acent a gap between teeth, the second sensor not being adjacent an edge of a tooth when the first sensor is adjacent an edge of a tooth.
4. A starting system as claimed in claim 3, in which the teeth are equally spaced around the wheel, with the gaps between the teeth being equal to the width of the teeth.
5. A starting system as claimed in claim 4, in which the first and second sensors are spaced from each other by an integer and a half times the tooth width.
6. A starting system for an engine as claimed in any of the preceding claims, in which the control unit is fbrther adapted to open an exhaust port if the engine is rotating in the reverse direction.
7. A starting system for an engine as claimed in any of the preceding claims, in which the control unit is further adapted to disengage a starter clutch if the engine is rotating in the reverse direction.
8. A starting system for an engine as claimed in any of the preceding claims, in which a signature tooth is provided on the toothed wheel for enabling sensing of engine position.
9. A starting system for an engine as claimed in any of the preceding claims, in which the first and second detector are spaced apart around the toothed wheel by around 180 degrees.
10. A starting system as claimed in any of the preceding claims, fitted to an internal combustion engine.
11. A starting system as claimed in claim 10, in which the engine is a spark-ignition engine.
12. A starting system as claimed in claim 10 or claim 11, fitted to the engine of an unmanned aerial vehicle.
13. A starting system for an engine substantially as described herein, with reference to and as illustrated in Figures 1 to 3b of the accompanying drawings.